import java.util.LinkedList;
import java.util.Queue;

public class Test {
    public static void main(String[] args) {
        //
    }
}

class Solution
{
    int[] dx = {0, 0, -1, 1};
    int[] dy = {1, -1, 0, 0};

    public int[][] updateMatrix(int[][] mat)
    {
        int m = mat.length, n = mat[0].length;
        // dist[i][j] == -1：标记当前位置没有被搜索过
        // dist[i][j] != -1：存的是最短距离
        int[][] dist = new int[m][n];
        for(int i = 0; i < m; i++)
            for(int j = 0; j < n; j++)
                dist[i][j] = -1;
        Queue<int[]> q = new LinkedList<>();
        // 1. 把所有的源点加⼊到队列⾥⾯
        for(int i = 0; i < m; i++)
            for(int j = 0; j < n; j++)
            {
                if(mat[i][j] == 0)
                {
                    dist[i][j] = 0;
                    q.add(new int[]{i, j});
                }
            }

        // 2. ⼀层⼀层的往外扩
        while(!q.isEmpty())
        {
            int[] t = q.poll();
            int a = t[0], b = t[1];
            for(int i = 0; i < 4; i++)
            {
                int x = a + dx[i], y = b + dy[i];
                if(x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {
                    dist[x][y] = dist[a][b] + 1;
                    q.add(new int[]{x, y});
                }
            }
        }
        return dist;
    }
}


